1. Field of the Invention
The present invention relates to improved backing materials for flexographic printing plates. More particularly, the invention relates to relatively high density materials with discrete, separated closed cell encapsulated voids having improved properties when used as a means for mounting a flexographic printing plate on a printing cylinder.
2. Background Art
Flexography applies broadly to printing processes utilizing flexible substrates bearing elastomeric or rubbery relief printing surfaces. It requires less unit area pressure to effect good ink transfer than other processes. Examples of such printing plates are set forth in detail in U.S. Pat. No. 5,015,556 issued to Martens on May 14, 1991.
Historically, to mount the plates to the printing cylinder, double adhesive coated vinyl sheets commonly referred to as "sticky back" have been used. These sheets are incompressible and tend to vary in caliper up to 0.002 to 0.003 inch. The plate, plate cylinder and impression cylinder also each have variations in surface height or thickness. Thus, in a worst case scenario, if all of the plus and minus variations happen to fall in the same areas, the total variation may be as much as 0.004 to 0.008 inch. Such inaccuracies dictate the use of increased pressure in the printing process, but such increased pressure causes a deterioration in print quality due to yielding under pressure of the printing plates. Undesirable results include a dirty appearance of the printing and inaccurate reproduction of half tones. Thus a need exists for backing materials having more uniform thickness. This need has also been increased due to the increased use of thinner plates produced by laser engraving or photopolymerization techniques.
It has also been proposed to use layers of synthetic polymeric foams as backing materials in an effort to overcome these shortcomings. See for example, U.S. Pat. No. 3,903,794 issued to Grupe et al. on Sep. 9, 1975 in which the use of a composite cushioning member is formed from a foam and a fabric backing. Other composite materials are shown, for example, in U.S. Pat. No. 4,303,721 issued to Rodriguez on Dec. 1, 1981. Other foam-shaped materials are shown, for example, in U.S. Pat. No. 4,574,697 issued to Feeley on Mar. 11, 1986.
Even in light of these various available backings, a need has continued to exist for improved materials that will cushion a printing plate to allow flexing of the printing surface during the printing process but yet which would rebound rapidly to substantially the original thickness within a fraction of a second so that high quality prints can be made repeatedly.
Composite and foam materials hitherto available have suffered either from lack of deformability, or if they are readily deformable, from a lack of sufficient resiliency to rebound rapidly enough and repeatedly to the original dimensions. Most such materials also are not sufficiently accurate in caliper.
To compensate for these problems it is currently common practice to use composite and foam materials for printing half tone portions of the copy being reproduced but not for the solid portions of the copy, as they lack sufficient density and caliper accuracy to reproduce solids without a grainy or incomplete (snowflake) appearance.
Briefly summarized, the invention provides an improved cushioning sheet material adapted to be positioned between a flexographic printing cylinder and a flexible printing plate. The cushioning sheet material has a uniform thickness to within about 0.00025 to 0.0005 inch of the desired thickness and a density greater than 40 pounds per cubic foot. It is sufficiently deformable to experience a reduction in thickness of 6 to 9 percent when subjected to a compressive force of 45 psi and has a resiliency sufficient to recover at least 99.5 percent of its original thickness within 0.1 second after the compressive force is removed.